Interrupts on 8086 microprocessor by vijay kumar.k

information about Interrupts on 8086 microprocessor

1. Interrupts on 8086 Microprocessor By Vijay Kumar. K Dept. of ECE JNTUACEK

2. When your phone rings during a lecture, what will happen? When you are studying then your cell phone rings – what will you do? When you finish talking on the phone then you will continue with your study. When being interrupted, you will perform some pre-defined action. Interrupt has priority – some interrupt is more important than the others. For example, answering your phone is more important than opening the door. Now your phone rings again and someone also knocking at your door then what will you do?

3. Introduction An interrupt is used to cause a temporary halt in the execution of program. The meaning of ‘interrupts’ is to break the sequence of operation. While the Microprocessor is executing a program, an ‘interrupt’ breaks the normal sequence of execution of instructions, diverts its execution to some other program called Interrupt Service Routine (ISR). After executing ISR, IRET returns the control back again to the main program. Interrupt processing is an alternative to polling.

4. Need for Interrupt: Interrupts are particularly useful when interfacing I/O devices, that provide or require data at relatively low data transfer rate. Three types of interrupts sources are there: An external signal applied to NMI or INTR input pin( hardware interrupt) Execution of Interrupt instruction( software interrupt) Interrupt raised due to some error condition produced in 8086 instruction execution process. (Divide by zero, overflow errors etc) Sources of Interrupts:

5. 8086 Interrupt Sources An 8086 interrupt can come from any one of the following three sources: An external signal applied to the non- maskable interrupt (NMI 17 pin) pin or to the interrupt (INTR 18 pin) pin. An interrupt caused by a signal applied to one of these inputs is called hardware interrupt . The execution of the instruction INT n , where n is the interrupt type that can take any value between 00H and FFH. This is called software interrupt . An error condition such as divide-by-0 , which is produced in the 8086 by the execution of the DIV/IDIV instruction or the trap interrupt.

6. How to get key typed in the keyboard or a keypad ? Polling :- The CPU executes a program that check for the available of data, If a key is pressed then read the data, otherwise keep waiting or looping!!! Interrupt:- The CPU executes other program, as soon as a key is pressed, the Keyboard generates an interrupt. The CPU will response to the interrupt read the data. After that returns to the original program. So by proper use of interrupt, the CPU can serve many devices at the “same time”

7. Polling vs Interrupt The keyboard controller can hold only a single keystroke. Therefore, the keyboard controller must be freed before the next keystroke arrives. The keystroke is passed to the CPU by putting it in the keyboard buffer. So, the keyboard controller keeps on passing the keystroke input to the CPU, But how does the CPU attend to it? The CPU is not at the disposal of the keyboard controller; it is usually busy doing several other operations. So, we need some mechanism to indicate to the CPU that a keystroke has arrived . How is this done? There are two approaches to making sure that the CPU pays attention: Polling-based Interrupt-based

8. Example: Polling Vs Interrupt Keystroke causes interrupt

9. Figure 2: Polling-based interrupt handling Polling Based System:- The CPU executes a program that check for the available of data If a key is pressed then read the data, otherwise keep waiting or looping!!!

10. Keystroke passed to the CPU No keystroke for CPU

11. Interrupt-based approach Interrupt-based systems The CPU executes other program, as soon as a key is pressed, the Keyboard generates an interrupt . The CPU will response to the interrupt – read the data. After that returns to the original program. So by proper use of interrupt, the CPU can serve many devices at the “same time”

12. Example of interrupt How to control a robot that has sensors to detect obstacles and makes a turn Controlling a robot by using Polling & Interrupt

13. Interrupt Keeping moving until interrupted by the sensor Interrupt received then do pre-defined operation After finishing the interrupt service return to normal operation i.e keep moving forward again Polling Move forward in a pre-defined unit Check sensor reading Do nothing if no obstacle or turn if obstacle detected Loop back and move forward again

14. Polling vs Interrupt Control of a robot Move forward Check sensor Y Stop or turn Move forward interrupt N obstacle

15. How would the processor work with an Interrupt ? How would the processor work with an Interrupt

16. Interrupts Software Interrupts INT n Hardware I nterrupts Maskable Interrupts Nonmaskable Interrupts The programmer can choose to mask specific interrupts and re-enable them later The programmer cannot control when a non maskable interrupt is served The processor has to stop the main program to execute the NMI Service Routine. 256 Types of software Interrupts INT 00 to INT FF

17. Executes the INT instruction Interrupts the INT instruction during the assembly time Moves the INT instruction to the Vector Table Vector Table occupies location 00000H to 0003FFh of the program memory. It contains the code segment (CS) and I nstruction Pointer (IP) for each kind of interrupt. Processing of Interrupt by the Processor

18. Pushes the content of the flag register onto the stack to preserve the status of the interrupt (IF) and trap flags (TF), by decrementing the stack pointer (SP) by 2 Disables the INTR interrupt by clearing IF in the flag register Resets TF in the flag register, to disable the single step or trap interrupt Pushes the content of the code segment (CS) register onto the stack by decrementing SP by 2 Pushes the content of the instruction pointer (IP) onto the stack by decrementing SP by 2 Performs an indirect far jump to the start of the interrupt service routine (ISR) corresponding to the received interrupt. If an interrupt has been requested, the 8086 Microprocessor processes it by performing the following series of steps: 8086 Interrupt Processing

19. Steps involved in processing an interrupt instruction by the processor Jumps to the Interrupt Vector Table Takes the CS and IP in the Vector Table Pushes the existing CS and IP on the Stack Executes the Interrupt instructions Loads the new CS and IP Jumps to the Interrupt Service Routine Comes back and continues the Main Program Executes the Interrupt Service Routine

20. Interrupt Push flags register Clear IF and TF Push CS and IP Load CS and IP Pop IP and CS Pop flags register Interrupt Service Routine (ISR) Interrupt program : : : : : : IRET Main Program Processing of an Interrupt by the 8086

21. RESET as a Non- maskable Interrupts

22. Process sequence in the processor Completes the current instruction that is in progress Pushes the Flag Register values on to the Stack Pushes the CS value and IP value of the return address on to the stack IP is loaded from contents of the word location 00008H CS is loaded from contents of next word location 0000AH Interrupt Flag and Trap Flag are reset to 0.

23. 1 KB CS IP Type 01H Interrupt (Trap or Single step) Type 02H Interrupt (NMI) Type 03H Interrupt (Break Point) Type 04H Interrupt (Over Flow) Type 20H Interrupt (Available) Type 21H Interrupt (Available) Type 1FH Interrupt (Reserved) Type FFH Interrupt (Available) Type 05H Interrupt (Reserved) Type 00H Interrupt ( Divide by Zero) Dedicated Interrupts (05) Reserved Interrupts (27) Available Interrupts (224) CS IP 003FFH 00084H 00080H 0007CH 00014H 00010H 0000CH 00008H 0004FH 0003FH 00002H 00001H 00000H 003FCH IVT:- Interrupt Vector Table

24. Given a vector, where is the ISR address stored in memory ? Offset = Type number X 4 Example :- INT 02h Offset = 02 x 4 = 08 = 00008h Type 0 or INT 00 Interrupt 00000H 00001H 00002H 00003H 2 bytes 2 bytes CS IP CS LSB MSB CS LSB CS MSB IP LSB IP MSB

25. 256 Interrupts Of 8086 are Divided in To 3 Groups Type 00 to Type 04 interrupts - These are used for fixed operations and hence are called dedicated interrupts Type 05 to Type 31 interrupts Not used by 8086,reserved for higher processors like 80286 80386 etc. Type 32 to Type 255 interrupts Available for user, called user defined interrupts these can be H/W interrupts and activated through INTR line or can be S/W interrupts.

26. Type – 0 :- Divide by Zero Error Interrupt Quotient is large cant be fit in al/ax or divide by zero Type –1:- Single step or Trap Interrupt Used for executing the program in single step mode by setting trap flag. Type – 2:- Non- Maskable Interrupt This interrupt is used for executing ISR of NMI pin (positive edge signal ), NMI can’t be masked by S/W. Type – 3:- One-byte INT instruction interrupt Used for providing break points in the program Type – 4 Over flow Interrupt Used to handle any overflow error after signed arithmetic.

27. BIOS Interrupt or Function Calls The BIOS (basic input/output system) is boot firmware, which is designed to be the first program run by a PC when powered on. The initial function of the BIOS is to identify, test, and initialize system devices such as the video display card, hard disk, floppy disk, and other hardware. The BIOS prepares the machine for a known state, so that the software stored on the compatible media can be loaded, executed, and given control of the PC. BIOS function calls , also known as BIOS interrupts, are stored in the system ROM and in the video BIOS ROM present in the PC.

28. INT 21h is provided by DOS. When MS-DOS is loaded into the computer, INT 21H can be invoked to perform some extremely useful functions. These functions are commonly referred to as DOS INT 21H function calls. Data input and output through the keyboard and monitor are the most commonly used functions. DOS interrupts

29. Display the message defined with variable DATA DB ‘Microprocessor,’$’ MOV AH,09 Option 9 to display string of data MOV DX, OFFSET DATA DX = offset address of data INT 21H Invoke the interrupt 2. Inputting a single character, with echo. MOV AH, 01 Option 01 to input one character INT 21H Invoke the interrupt Below are two examples that use DOS interrupts.

30. Function call 01: Read the key board Input parameter AH = 01 read a character from keyboard. Echo it on CRO screen and return the ASCII code of the key pressed in AL output parameter: AL = ASCII code of character Function call 02h: Display on CRT screen Input parameter: AH = 02 Dl = ASCII character to be displayed on CRT screen Function call 03: Read character from com1 Input parameter: AH = 03h Function: Reads data from com port Output parameter: AL = ASCII code of character Function call 04: Write character to com1 Input parameter: AH = 04h DL = ASCII code of character to be transmitted Function: Writes data to com port DOS (Disk Operating System) Interrupts

31. Function call 05: Write to Lpt1 Input parameter: AL = 05H DL = ASCII code of character to be printed Function: Print the character available in dl on printer attached to Lpt1 Function call 09: Display a character string Input parameter: AH = 09, DS:DX= A ddress of character string Function: Displays the characters available in the string to CRT till a $ symbol Function call 0Ah: Buffered key board input Input parameter: AH = 0Ah DS:DX = Address of keyboard input buffer Function: The ASCII codes of the characters received from keyboard are stored in keyboard buffer from 3 rd byte. 1 st byte of buffer = size of buffer upto 255. It receives the characters till specified no.Of characters are received or enter key is presses which ever is earlier

32. INT 10H subroutines are burned into the ROM BIOS of the 8086 based and compatibles and are used to communicate with the computer user screen video. Much of the manipulation of screen text or graphics is done through INT 10H. Among them are changing the color of characters or background, clearing screen, and changing the locations of cursor. Below are two examples that use BIOS interrupts. Clearing the screen: MOV AX, 0600H ;scroll entire screen MOV BH, 07 ;normal attribute MOV CX, 0000 ;start at 00,00 MOV DX, 184FH ;end at 18, 4F INT 10H ;invoke the interrupt BIOS interrupt

33. BIOS (Basic Input/output System) INTERRUPTS Function Call 00: Select Video Mode Input Parameter: AL = Mode Number AH = 00h Function: It Changes The Display Mode And Clears The Screen AL = 00 40 X 25 Black And White AL = 04 320 X 200 Color AL = 10h 640 X 350 X 16 Color b) Function Call 03: Read Cursor Position Input Parameter: AH = 03 BH = Page Number Function: Reads Cursor Position On Screen Output Parameters: CH = Starting Line CL = Ending Line DH = Current Row DL = Current Column C) Function Call 0E : Write Character On CRT Screen And Advance Cursor Input Parameter : Ah = 0Eh AH = ASCII Code Of The Character BH = Page(text Mode) BH = Color(graphics) Function: Display Character Available In Al On Screen INT 10H:Video Service Interrupt :- It Controls The Video Display

34. INT 11H:- Determine the type of equipment installed. Register AX should contain FFFF H and instruction INT 11H to be executed. On return, register AX will indicate the equipment's attached to computer INT14H:- Control the serial communication port attached to the computer. Ah should contain the function call Function Call 00:initialize The Com Port Function Call 01: Send A Character Function Call 02:receive A Character INT 16H:- Keyboard interrupt AH should contain the function call Function call 00: Read keyboard character, It will return ASCII code of the character Function call 01: Get key board status Other Interrupts:-